Radiation can have harmful effects on microelectronics. For years, practitioners have studied the various ways that different types of radiation affect microelectronics, and have attempted to devise ways of eliminating or at least mitigating the problems that these various types of radiation can create for microelectronics. Three major types of ionizing radiation-induced effects are soft errors (a.k.a. single event effects), dose-rate effects, and total dose effects. Other non-ionizing radiation effects are also well-documented.
Single event effects occur when a high energy particle (such as a cosmic ray, proton, or neutron) changes the state of a particular device in an integrated circuit, thereby causing a loss of information. Single event effects are typically localized to a particular region of an integrated circuit.
Dose rate effects are caused by the exposure of an entire integrated circuit to a flood of radiation, typically X-ray or Gamma-ray radiation. Dose rate effects are typically related to a short burst (ns to ms) of high intensity radiation, such as that emitted by a nuclear detonation. Such exposure can cause temporary, and in some cases permanent, failure in integrated circuits.
Total dose effects in devices are related to the permanent failure of an integrated circuit caused by an accumulation of radiation dose. Such failures typically result from the trapping of holes produced by ionizing radiation in an insulating SiO2 region, such as in a gate oxide or field oxide region. As the name suggests, total dose effects are related to the entire exposure history of an integrated circuit and when the total dose exceeds some threshold value, circuit failure is observed. This cumulative nature of total dose effects distinguishes this type of radiation effect from single event effects and dose rate effects, which are related instead to short term, transient phenomena.
Total dose effects can cause shifts in device characteristics, such as shifts in threshold voltage, transconductance, saturation current, and so on. The threshold voltage of a Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) is usually defined as the gate voltage at which a depletion region forms in the substrate (body) of a transistor. In an n-type MOSFET (NMOS), the substrate of the transistor is composed of p-type silicon, which has more positively charged electron holes compared to electrons. When a voltage is applied to the gate, an electric field causes the electrons in the substrate to become concentrated at the region of the substrate nearest the gate causing the concentration of electrons to be equal to that of the electron holes, creating a depletion region.
If the gate voltage is below the threshold voltage, the transistor is turned off and ideally there is no current from the drain to the source of the transistor. If the gate voltage is larger than the threshold voltage, the transistor is turned on, due to there being more electrons than holes in the substrate near the gate, creating a channel where current can flow from drain to source. Shifts in the threshold voltage can detrimentally change the operating characteristics of the transistor.
Bias effects, such as negative bias temperature instability (NBTI), can also cause shifts in threshold voltage. NBTI occurs as a result of stressing a device with a large negative bias at elevated temperatures. The NBTI-induced threshold voltage shift typically occurs over a period of months or years, depending on the operating conditions of the device. NBTI is most problematic for high-performance or high-reliability devices, and analog/mixed-signal devices are more susceptible than digital devices.
Shifts in device characteristics caused by total ionizing dose radiation or bias effects are undesirable. Thus, it would be beneficial to have a circuit architecture that mitigates this problem.